This invention pertains to the application of fluorinated urethane compositions to stone in order to protect the stone from the deleterious effects of water and pollution. Preservation of historic monuments, buildings, sculptures is one object of the present invention. Provision of weather and pollution resistant stone construction materials is another object of the present invention.
It has long been recognized that a combination of man-made and natural atmospheric factors are having deleterious effects on stone structures including many monuments of considerable artistic and historical importance. A variety of efforts have been undertaken over the years to identify ways to protect and preserve these structures, but these efforts have met with only partial success. Most recently, a well-organized and concerted effort has been undertaken by Piacenti and coworkers, with sponsorship from the Target Project for the Cultural Heritage of the Consiglio Nazionale della Recerche of Rome, Italy, and represents the current state of the art.
Water, in the form of both rainfall and condensation, is the primary material of concern, although organic matter may be of secondary concern. For example, all building materials are subject to stress and concomitant cracking resulting from the penetration of water into the structure of the material followed by cycles of freezing and thawing. Also, water in combination with CO2, which occurs naturally, and nitrous and sulfurous gases, which are man-made pollutants, forms acids which rapidly eat away at the stone.
A successful attack on the problem will necessitate some tradeoffs. While it is highly desirable to minimize the contact between water and stone, by achieving maximum water repellency, it is also necessary to provide high water vapor permeability in order to permit venting of that water which finds its way into the microstructure of the stone. Substances with high permeability to water vapor are often not those of the highest water repellency. High resistance to acid and abrasion are also of considerable importance. Furthermore, cost of materials is a factor in any practical application. The smaller the amount of material required to achieve the desired effect, the better.
There are other tradeoffs. For example, it is particularly desirable that the coating material coat but preferably not block the pores. To achieve this, a coating must be applied with viscosity in a range which permits wetting of the pores via capillary action. High wetting is also required to ensure thorough and uniform coverage. However, the coating must be provided with sufficient adhesion to the outside surface upon which it is deposited that at least some amount will remain thereon.
Other requirements for such materials include chemical inertness, low volatility, photooxidative stability, thermal stability, sufficient solubility in environmentally friendly solvents to permit removal at some future date. The coating must also be clear and colorless, and remain so for its lifetime. And it should be susceptible to dissolution in environmentally friendly solvents for purposes of application. In the current state of the art, the application solvent of choice is supercritical CO2, as described in Carbonell et al., WO 99/19080.
In a series of patents, U.S. Pat. Nos. 4,499,146, 4,746,550, 4,745,009, 4,902,538, Piacenti et al. disclose compositions based upon perfluoropolyethers having molecular weights in the range of 500-5000 for use in the protection of stone from the effects of water and atmospheric pollutants. In the art of Piacenti, excellent combinations of water repellency and water vapor permeability are achieved.
In U.S. Pat. No. 4,902,538, good results are achieved in compositions having highly crystalline particles of polytetrafluoroethylene and copolymers thereof intermixed with the perfluoropolyethers. However, when stone of porosity of greater than ca. 30% is treated, impractically high levels of coating material are required to achieve the desired coverage with the desired water repellency. Levels in the range of at least 150 g/m2 are disclosed, more than 10 times the amount required for low-porosity marble. The effect of this high coating level on permeability is not disclosed. Its effect on cost, however, is clearly undesirable. Furthermore, use of highly crystalline polymers, such as polytetrafluoroethylene, is undesirable because, unless they are sintered at high temperatures, they will be too readily susceptible to removal from the treated surface by abrasion and erosion. Further still, they are not readily soluble in the delivery medium of choice, CO2, or any other desirable medium.
Also disclosed in the art in Piacenti et al., U.S. Pat. No. 4,764, 431, are copolymers of vinylidene fluoride which are less effective than the perfluoropolyethers.
Fluorinated acrylic polymers are disclosed by Ciardelli et al., Prog. in Org. Coatings, 32, 43-50 (1997). The polymers disclosed therein are characterized by hydrocarbon backbones and fluorinated pendant groups. These polymers exhibit similar functionality to the perfluoropolyethers.
Guidetti et al. disclose the use of polyfluorourethanes for protecting stone in xe2x80x9cPolyfluorourethanes as stone protectivesxe2x80x9d, 7th International Congress on Deterioration and Conservation of Stone, 1053-62 (1992).
There is considerable incentive in the art to discover new materials which possess several of the above attributes desired for the application.
The present invention provides a process for protecting stone comprising contacting stone with a non-polymeric composition having the formula
(R2O2CNH)pR1NHCOxe2x80x94(OCHR3CH2)mxe2x80x94Xnxe2x80x94Rf
where p=1 or 2, R1 is an aliphatic, cycloaliphatic or aromatic hydrocarbyl di- or tri-radical, R2 is a fluorinated or non-fluorinated hydrocarbyl or hydroxy-hydrocarbyl radical optionally substituted by one or more ether oxygens, R3 is hydrogen or alkyl, X is a diradical selected from the group consisting of xe2x80x94OCH2CH2xe2x80x94, xe2x80x94OCH2CH2N(R4)SO2xe2x80x94, xe2x80x94CH2xe2x80x94, xe2x80x94Oxe2x80x94, and xe2x80x94OCH2xe2x80x94, wherein R4 is alkyl, Rf is perfluoroalkyl, and m=0-30, n=0 or 1, with the proviso that if n=0 or if n=1 and X is xe2x80x94Oxe2x80x94, then mxe2x89xa00.
For the purpose of the present invention, the term xe2x80x9cstonexe2x80x9d means a natural stone used in construction or sculpture (such as granite, marble, limestone, or sandstone) as well as tile, cement, brick, stucco, and the like.
The method of the present invention provides surprising benefits over the methods of the art. In the method of the present invention, a slightly fluorinated non-polymeric urethane composition is employed as a coating agent for stone in order to provide high liquid moisture barrier, good moisture vapor permeability, and resistance to environmental pollutants. The non-fugitive, very low areal density coating formed on the stone surface is surprisingly effective over the materials of the art. Furthermore, the urethane of the present invention is readily soluble in a variety of solvents by virtue of its non-polymeric nature, and is thereby both readily applied in the form of an environmentally friendly solution and readily removed by conventional solvents should that be deemed necessary after application. Further still, the highly desirable effects of the method of the present invention are achieved employing a urethane containing relatively little of expensive fluorocarbon ingredients.
Suitable for the practice of the present invention are urethanes represented by the formula
(R2O2CNH)pR1NHCOxe2x80x94(OCHR3CH2)mxe2x80x94Xnxe2x80x94Rf
where p=1 or 2, preferably p=1. R1 is an aliphatic, cycloaliphatic or aromatic hydrocarbyl di- or tri-radical. Preferably R1 is a di-radical; more preferably, R1 is a cycloaliphatic diradical. R2 is a fluorinated or non-fluorinated hydrocarbyl or hydroxyhydrocarbyl radical optionally substituted by one or more ether oxygens. Preferably the urethanes are a mixture where R2 is Rf and/or alkyl. Most preferably R2 is Rf and/or methyl or ethyl. R3 is hydrogen or alkyl, preferably R3 is hydrogen. X is a diradical selected from the group consisting of xe2x80x94OCH2CH2xe2x80x94, xe2x80x94OCH2CH2N(R4)SO2xe2x80x94, xe2x80x94CH2xe2x80x94, xe2x80x94Oxe2x80x94, and xe2x80x94OCH2xe2x80x94, wherein R4 is alkyl. Rf is a perfluoroalkyl radical, and may in practice be a mixture of perfluoroalkyl radicals, m=0-30, n=0 or 1, with the proviso that if n=0 or if n=1 when X is xe2x80x94Oxe2x80x94, then mxe2x89xa00. Preferably, n=0, and m=1 to 20, most preferably m=1.
In a preferred embodiment, R1 is a cycloaliphatic diradical represented by the formula 
wherein and R5, R6, and R7 are all alkyl, preferably methyl.
The urethanes suitable for use in the present invention are known in the art and may be synthesized according to the methods described in Anton et al., U.S. Pat. No. 5,859,126, Anton, U.S. Pat. No. 5,637,657, and Anton et al., U.S. Pat. No. 5,605,956.
In a preferred embodiment, the fluorinated organic urethane is an adduct of a fluorinated monofunctional alcohol, a non-fluorinated organic polyisocyanate and a non-fluorinated organic alcohol. Suitable polyisocyanates include aromatic, aliphatic and cycloaliphatic di and trifunctional polyisocyanates such as 1,6-hexamethylene diisocyanate, isophorone diisocyanate, 4,4xe2x80x2-biphenylene diisocyanate, toluene diisocyanate, bis cyclohexyl diisocyanate, tetramethylene xylene diisocyanate, ethyl ethylene diisocyanate, 2,3-dimethyl ethylene diisocyanate, 1-methyltrimethylene diisocyanate, 1,3-cyclopentylene diisocyanate, 1,4-cyclohexylene diisocyanate, 1,3-phenylene diisocyanate, 1,5-naphthalene diisocyanate, bis-(4-isocyanatocyclohexyl)-methane diisocyanate, 4,4xe2x80x2-diisocyanatodiphenylether diisocyanate and the like.
Typical trifunctional isocyanates that can be used are triphenylmethane triisocyanate, 1,3,5-benzene triisocyanate, 2,4,5-toluene triisocyanate and the like. Oligomers of diisocyanates also can be used such as the trimer of hexamethylene diisocyanate which is sold under the tradename xe2x80x9cDesmodurxe2x80x9d N. Preferably, aliphatic diisocyanates would be used and most preferably cycloaliphatic diisocyanates.
Preferred fluorinated monofunctional alcohols are represented by the formula
Rfxe2x80x94(X)nxe2x80x94(CH2CHR3xe2x80x94O)mxe2x80x94H
where Rf is preferably a straight chain or branched chain fluoroalkyl or chlorofluoroalkyl group having 4-20 carbon atoms optionally substituted by ether oxygen. Preferably, Rf is a perfluoroalkyl group having 4-20 carbon atoms and most preferably, Rf is a perfluoroalkyl group containing 6-12 carbon atoms. X is a divalent radical, preferably xe2x80x94CH2CH2Oxe2x80x94, xe2x80x94SO2N(R4)CH2CH2Oxe2x80x94, xe2x80x94CH2xe2x80x94, xe2x80x94Oxe2x80x94, or xe2x80x94CH2Oxe2x80x94 where R4 is an alkyl group preferably having 1-4 carbon atoms. R3 is H or an alkyl group having 1-4 carbon atoms, H and methyl being preferred. n is 0 or 1 and m is 0-30, provided that if n is 0 or if n=1 and X is xe2x80x94Oxe2x80x94, then m must be greater than or equal to 1; preferably n=0 and m is 1 to 20. Most preferably, m=1.
Preferred fluorinated monofunctional alcohols are represented by one of the following formulae:
Rfxe2x80x94(CH2CH2xe2x80x94O)mH
where Rf is a perfluoroalkyl group having 6 to 12 carbon atoms optionally substituted by ether oxygen and m is 5 to 15;
Hxe2x80x94(CF2CF2)mxe2x80x94CH2OH
where m is 1-6;
C8F17xe2x80x94SO2xe2x80x94N(R5)xe2x80x94(CH2CH2O)mxe2x80x94H
where R5 is an alkyl group having 1-4 carbon atoms and m is 1-30, preferably m is 1-20;
CF3xe2x80x94(Oxe2x80x94C(CF3)(F)CF2)pxe2x80x94Oxe2x80x94(CH2CH2O)mxe2x80x94H
where n is 0-10 and m is 1-20; and
Rfxe2x80x94CH2CH2xe2x80x94OH
where Rf is described above.
A number of fluorinated alcohols are available commercially as Zonyl(copyright) Fluorotelomer Intermediates from E.I. du Pont de Nemours and Company, Wilmington, Del.
Other perfluoroalkyl alcohols can also be used in the present invention, such as 2-N-methyl-N-ethanolperfluorooctane sulfonamide, available commercially from Dainippon Ink and Chemical, Inc., Tokyo 103, Japan. Fluorinated diols prepared by the procedure of U.S. Pat. No. 4,946,992 and fluorinated thiols prepared as in U.S. Pat. No. 3,544,663, in particular Example 1 therein, are also suitable.
It will be understood by the practitioner of the art that the ingredients employed in the reaction to form the urethane of the present invention, and therefore the product urethane generated therefrom may be mixtures of one or more of the compounds hereinabove said to be suitable for the practice of the invention. In particular, the Zonyl(copyright) Fluorotelomer Intermediates preferred are in fact mixtures of a homologous series of species represented by the formula
CF3(CF2CF2)nCH2CH2OH
where n is 3 to 9.
The fluorinated organic urethane of the method of the present invention is prepared as taught in the art hereinabove cited, in which the fluorinated monofunctional alcohol, organic polyisocyanate and organic alcohol are charged into a reaction vessel optionally with solvents and a catalyst for about 0.1-4 hours and heated to about 50xc2x0-120xc2x0 C., preferably 60xc2x0-85xc2x0 C.
About 0.1 to 100 mole % of active isocyanates are reacted with the fluorinated alcohol. Preferably, greater than 40% of the isocyanates are reacted with the fluorinated alcohol and most preferably greater than 75%. The remaining isocyanates are reacted with monofunctional organic alcohols such as allyl or propyl alcohols.
Typical solvents that are used are alkyl acetates such as ethyl acetate, butyl acetate and the like, alkylene glycol alkyl ether acetates such as ethylene glycol, monobutyl ether acetate and the like and ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone.
Typical catalysts that are used are organo tin containing catalysts like alkyl tin laurates such as di-n-butyl tin dilaurate, dibutyl tin di-2-ethylhexoate, stannous octoate, stannous oleate and the like.
In the practice of the present invention, one or more of the urethanes hereinabove described are applied by any convenient method to the surface of the stone which is to be protected from the effects of water, such as rainfall, and environmental pollutants. Foremost, the coating must provide a barrier to liquid water with minimal effect on the natural water vapor permeability of the stone. One way of achieving this is to provide durable coating in as thin a layer as possible on the wall surface of each pore of the stone without actually filling or blocking the pore using a material of the lowest possible surface tension. Coating materials which exhibit a desirable combination of properties are characterized by pendant groups comprising perfluoralkyl functional groups in sufficient concentration that the surface presented to incident liquid water such as rainfall is characterized by a high density of the perfluorinated groups and a consequently very low surface tension. In the most preferred embodiment of the urethane of the method of the present invention, the perfluoralkyl group in Rf as hereinabove defined has at least 7 carbons in a linear chain. The resulting low surface tension achieved by the application of the preferred urethane to a porous stone surface is the source of the thermodynamic driving force for complete wetting of the pores as well as liquid water repellency. To reduce the kinetic barrier to complete pore wetting, the viscosity should be as low as possible. This represents a particularly desirable attribute of the method of the present invention because the urethane employed in the method of the present invention is a non-polymeric liquid which readily forms low viscosity solutions in a number of convenient solvents.
While in no way limiting the scope of the invention, it is estimated that the viscosity of the coating during application of the coating to the stone is preferably less than about 10 Pa-s to achieve optimum coating performance. It will be obvious to one of skill in the art that while it is desirable to employ materials which afford low viscosity solutions, usually associated with low molecular weight or non-polymeric materials, the materials so employed cannot be of such low molecular weight that they evaporate from the stone surface. Until the discovery of the method of the present invention, all materials in the art employed for application to stone for the purpose of preservation have been polymeric.
It is further preferred that polar groups such as urethanes should be present in the coating material to promote adhesion of the coating material to the stone surface and decrease the tendency of the coating material to continually penetrate to the interior of the stone and reducing surface efficacy in terms of liquid water repellency. Esters, amides, and xe2x80x94CH2CF2xe2x80x94 moieties are examples of other such adhesion-promoting polar groups.
While according to the method of the present invention it is possible to apply the urethane to the stone in neat chemical form, using methods such as are known in the art such as painting or spraying, it is preferred to dissolve the urethane in a solvent which acts as a volatile diluent in the spraying operation to afford fast penetration at the early stages of coating while providing a high degree of control over the viscosity, the uniformity of coating and the coating thickness.
Solvents suitable for the practice of the present invention include acetone, methyl-ethyl ketone, ethyl acetate, t-butyl acetate, methyl isobutyl ketone (MIBK), hydrochlorofluorocarbons, perfluorocarbons. In the most preferred embodiment, the urethane of the present invention is dissolved in supercritical CO2 according to the methods described in Carbonell et al., WO 99/19080 or in the alternative in U.S. Pat. Nos. 4,923,720; 5,108,799; 5,290,603; and 5,290,604. Methyl isobutyl ketone is a preferred solvent for malting the urethanes as well as for diluting it to a suitable viscosity for absorption into stone within a practical time period. A preferred concentration of MIBK is urethane is 10% to 20% by weight.
These materials would preferably be sprayed from CO2 solutions of 75 weight % or less polymer at 40 to 70xc2x0 C., 2000 to 4000 psi. To promote material absorption into the stone it might also be preferable to add up to about 40 weight % acetone, t-butyl acetate, Oxol 100 (4-chlorobenzotrifluoride), or other such environmentally friendly diluent to the urethane.
It will be understood by one of skill in the art that numerous chemical compounds have been identified which may serve as the supercritical fluid for the urethane coating composition of the invention. However, CO2 is by far the preferred compound because of the low cost, low toxicity, ready formation of a supercritical fluid, and low environmental impact.
The urethane component of the coating composition is generally present in amounts ranging from 1 to 80 weight percent, based upon the total weight of the coating composition. Preferably, the urethane component should be present in amounts ranging from about 15 to about 55 weight percent on the same basis.
The supercritical fluid diluent should be present in such amounts that a liquid mixture is formed that possesses such a viscosity that it may be applied as a liquid spray. Generally, this requires the mixture to have a viscosity of less than about 300 centipoise at spray temperature. Preferably, the viscosity of the mixture of components ranges from about 5 centipoise to about 150 centipoise. Most preferably, the viscosity of the mixture of components ranges from about 10 centipoise to about 50 centipoise.
The supercritical carbon dioxide fluid is most preferably present in amounts ranging from about 45 to about 85 weight percent on the total compositional weight, thereby producing a mixture having viscosities from about 10 centipoise to about 50 centipoise at spray temperature.
While the art of applying coatings from supercritical CO2 teaches the use of an organic solvent in combination with a polymeric coating material and supercritical CO2, it is a particularly useful aspect of the present invention that the preferred urethane composition of the invention is a liquid at room temperature, and it is not necessary to form a low-viscosity solution or dispersion for mixing with the CO2. It is however optional to add a third component to the coating composition of the invention, the third component comprising one or more organic solvents employed for the purpose of improving viscosity control during spraying and xe2x80x9claydownxe2x80x9d of the coating material on the stone.
The organic solvents suitable for the practice of the most preferred embodiment of the invention generally include any solvent or mixture of solvents that is miscible with CO2, is a good solvent for the urethane, and is fugitive at the temperature at which the coating is being applied to the stone, normally at temperatures of about 0xc2x0 C. or above. Preferably, the solvent is also environmentally friendly. Suitable organic solvents include acetone, methyl-ethyl ketone, methyl isobutyl ketone, ethyl acetate, t-butyl acetate, hydrochlorofluorocarbons, and perfluorocarbons with acetone, methyl-ethyl ketone, ethyl acetate and t-butyl acetate preferred.
The coating composition of the invention is sprayed onto a substrate to form a liquid coating thereon by passing the liquid mixture under pressure through an orifice into the environment of the substrate to form a liquid spray.
Spray orifices, spray tips, spray nozzles, and spray guns used for conventional airless and air-assisted airless spraying of coating formulations such as paints, lacquers, enamels, and varnishes, are suitable for spraying the coating composition of the present invention. The spray pressure used in the practice of the present invention is a function of the specific coating formulation. In the case of supercritical fluid solutions, the minimum spray pressure is at or slightly below the critical pressure of the supercritical fluid. Generally the pressure will be below 5000 psi. Preferably, the spray pressure is above the critical pressure of the supercritical fluid and below 3000 psi. If the supercritical fluid is supercritical carbon dioxide fluid, the preferred spray pressure is between 1070 psi and 3000 psi. The most preferred spray pressure is between 1200 psi and 2500 psi.
The spray temperature used in the practice of the present invention is a function of the coating formulation. The minimum spray temperature is about 31xc2x0 C. The maximum temperature is determined by the thermal stability of the components in the liquid mixture. The preferred spray temperature is between 35xc2x0 C. and 90xc2x0 C. The most preferred temperature is between 45xc2x0 C. and 75xc2x0 C. Generally liquid mixtures with greater amounts of supercritical carbon dioxide fluid require higher spray temperatures.
One of skill in the art will recognize that the method of the present process, while specifically directed to the protection of stone, may equally be employed to apply coatings to a variety of substrates. Examples of suitable substrates include but are not limited to metal, wood, glass, plastic, paper, cloth, ceramic, masonry, stone, cement, asphalt, rubber, and composite materials.
Through the practice of the present invention, coatings may be applied to substrates in thicknesses of from about 0.5 to 100 micrometers. Preferably, the coatings have thicknesses of from about 1.0 to about 15 micrometers, while most preferably, their thicknesses range from about 1.5 to about 10 micrometers.
The method of the present invention provides a considerable benefit in that the urethane coating may be readily removed using inexpensive and environmentally benign solvents such as t-butyl acetate and acetone if it should be deemed desirable at some point in time following the application thereof.
The coatings on stone produced by the practice of the present invention are highly beneficial to the purpose of protecting the stone from environmental degradation. Two key attributes which are indicative of susceptibility to weathering are water absorption, typically by capillary action through the porous stone structure, and water vapor permeation rate. It is highly desirable that the water absorption of normally highly absorbent stone be reduced by as large a factor as possible, while water vapor permeability, normally high as well, be maintained at a high level. The coated stone of the present invention provides both high levels of water vapor permeability by virtue of the thin coatings which are found to be effective in providing the desired high resistance to water penetration.
The method of the present invention and the properties of the coated stone compositions provided thereby are further illustrated in the following specific embodiments.